The invention relates to an apparatus for the plasma treatment of substrates in a plasma discharge excited by radiofrequency between two electrodes supplied by a high-frequency source. The first electrode is configured as a hollow electrode. The second electrode bearing a substrate is placed in front of the hollow chamber of the first electrode and can be moved past the latter. At least the hollow electrode is surrounded by a dark space shield and has edges facing the second electrode, between which projections are provided, which are at the same potential as the first electrode.
Apparatus for the plasma treatment of substrates by radiofrequency commonly contain two electrodes, one of which is formed by the vacuum chamber and/or by the substrate holder, both consisting of metallic materials. It is known that a largely negative voltage is formed with respect to the plasma, which with respect to the surfaces acting as counterelectrode has the smaller active area. The negatively biased electrode is therefore regularly referred to, as it is herein, as the "cathode."
If a plate-like substrate holder is used, which is at the same potential as the vacuum chamber and thus represents the one electrode, and on the other hand a plate-like electrode opposite the substrate holder is used, then, on account of the area ratios necessarily created thereby, the negative potential forms on the latter electrode. Consequently material (=target) that is on this electrode (=cathode) becomes sputtered and precipitated onto the substrate. If it is desired to etch the substrates in such an apparatus, the substrates and target must be changed about accordingly.
If direct current is used, the polarity of the electrodes is necessarily determined by their connection to the particular pole of the direct-current source. German patent disclosure document 21 15 590 discloses the use, for example, of a hollow cathode with a rim raised toward the substrate holder in order to improve the uniformity of the coating thickness. This projecting rim, however, does not result in a reversal of the polarity if direct current is used.
German Patent 22 41 229 discloses an apparatus in which the use of radiofrequency brings it about that the hollow electrode, on account of its rim, has the larger area relative to the surface of the other electrode (substrate holder) directly facing the hollow chamber, and thus functions as an anode, so that, to a certain extent, a reversal of the conditions in regard to the bias takes place. This can be described, in simple terms, by saying that, if there is a sufficiently small distance between this electrode and the edge of the hollow electrode (gap S), the edge of the hollow electrode defines a discharge space in relation to which the metal parts of the apparatus outside of it no longer perform any electrode function, so that the conditions are determined only by the portions of the surface of the hollow electrode which face one another, on the one hand, and the substrate holder on the other. This can be termed the "rim effect."
A coating process of the kind described above is known (U.S. Pat. No. 4,767,641) in which the properties of the deposited coating, or those of the plasma-treated substrate surface, depend on the substrate bias (self-bias) formed between the plasma and the substrate surface. In the production of low-stress quartz coatings a voltage of definitely less than about 600 volts is necessary. In the production of carbon coatings the substrate bias (self-bias), however, must be greater than about 600 volts.
Accordingly, the problem to which the invention is addressed is to configure an apparatus for the plasma treatment of substrates in a plasma discharge, excited by radiofrequency, between two electrodes supplied by a radiofrequency source, of which the first is configured as a hollow electrode, such that the parameter of the self-bias will be variable in a simple manner, independently of other parameters such as radiofrequency, power and pressure.
This problem is solved in accordance with the invention by the fact that, between the projections lying between the edges, means are provided whereby the substrate self-bias is variable independently of the geometry of the discharge, the discharge pressure and the radiofrequency power. By this means the mobility of the electrons is reduced by a transverse magnetic field (H.ltoreq.100 KA/m) and to some extent approaches that of the ions, which are virtually unaffected by the magnetic field. The consequence is that the voltage of the marginal layer of the plasma is reduced, and with it the substrate self-bias. This effect is independent of the magnetic field strength, so that, in a surprisingly simple manner the substrate self-bias is variable independently of other parameters. In this manner, furthermore, an increase in the rate of deposition can be achieved. Advantageously, the magnets can also have different field strengths according to the work process. Also, an equalization of the plasma along the y-axis or lengthwise of the projections can thus be achieved. The apparatus according to the invention can advantageously be used as a hollow anode, and also as a hollow cathode. For the latter, the distance between the bottom of the electrode and the counterelectrode is varied accordingly. In this case the magnets bring it about that the plasma becomes denser and more uniform.